Increased mRNA and protein expression of extracellular matrix (ECM) components, including fibronectin, occurs during the development of glomerulonephritis and glomerulosclerosis in immunologically mediated kidney diseases. However, in addition to these quantitative changes in ECM expression, qualitative changes in these molecules may contribute to malformations in the composition of the glomerular matrix. These qualitative changes may include alterations in the splicing pattern of the V-region of fibronectin, since this region plays a role in its accumulation. The splicing patterns of this region have been studied in chronic graft-versus-host disease (GvHD) in mice, a model of lupus nephritis, and in chronic serum sickness (CSS) in rats, a model of immune complex nephritis. Cloning of the mouse fibronectin V-region from kidney tissue revealed 96.1 per cent homology with the corresponding domain in rat fibronectin. PCR (polymerase chain reaction) analysis of RNA from isolated glomeruli revealed three isoforms of this region in both mouse and rat fibronectin, namely inclusion or exclusion of the whole region, or exclusion of only the CS1 domain. In both models, increased exclusion of the V-region was observed early in the disease. However, in GvHD the splicing pattern returned to normal, whereas in CSS the shift persisted during the course of the experiment. Differentiated expression of fibronectin isoforms may exert an important effect on the structure and biological function of the glomerulus and may thus play a role in the development of glomerulonephritis and glomerulosclerosis.